US patent application No 2004/0174226 describes a power limiter circuit comprising a plurality of transmission line segments connected in succession. Each section consists of an inductor, coupled between the input and the output of the section and Schottky diodes connected anti-parallel (with mutually opposite polarity) between the output and ground. Different sections have different numbers of Schottky diodes. In the section nearest the output of the limiter circuit only two diodes are connected between the output of the section and ground, each in a respective polarity direction. In transmission line sections successively further upstream series connections of successively more diodes are connected between output of the section and ground.
In operation the Schottky diodes limit the power delivered to the output of the limiting circuit when a large power signal is input to the limiting circuit. In the case of a small power input signal the limiting circuit operates as a transmission line that passes the input signal to its output with a delay, without affecting the power.
US patent application No 2004/0174226 distinguishes from a prior limiter circuit with a resistor between its input and output and diodes connected anti-parallel between the output and ground. Such a circuit was said to be unsuitable for high frequencies due to distortion and losses.
By using a succession of sections the limiting function is distributed among all the diodes and by using increasingly more series connected diodes in the stages near the input of the limiting circuit it is avoided that the diodes received voltages greater than the limit imposed by semi-conductor manufacturing processes.
The circuit of US patent application No 2004/0174226 has the effect of reflecting the input signal when the power of the input signal is large. The circuit is provided with a circulator at the input of the limiter. The circulator directs the reflected power to the output of a power amplifier, where it is absorbed. This can be problematic at larger power levels. More generally, the need to handle reflections of large power input signals presents problems.
U.S. Pat. No. 3,568,099 describes a microwave limiter circuit wherein the problem of reflection during limiting operation is addressed. This circuit comprises a quarter wavelength transmission line section connected between its input and output, with a limiting diode connected between the output of the transmission line section and ground and a series connection of a limiting diode and a resistor connected between the input of the transmission line section and ground. The impedance of the resistor matches the impedance of the transmission line.
In operation, when the limiter circuit of U.S. Pat. No. 3,568,099 receives small signals, neither of the diodes in conductive, with the result that the circuit functions as a conventional transmission line section. When a large signal is applied, the diode at the output functions as a short circuit, which appears at the input as an open circuit, because the length of transmission line section is a quarter wavelength. The series connection of the diode and the resistor at the input of the transmission line determines the impedance of the limiter circuit for large signals: this is the matched impedance of the resistor, so that no signal is reflected.
The limiter circuit of U.S. Pat. No. 3,568,099 has the problem that the diodes can break down for large signals. If large diodes to counteract this problem, the increased capacitance of such diodes deteriorate matching, so that more power is reflected.
Limiters can be used for example in a phase-array radar system. In this case limiters are used to protect the sensitive input of the low-noise amplifiers (LNA) of T/R-modules inside the phase-array radar system. In phase-array radar system a circulator may be provided to direct transmission signals from a power amplifier to the antenna and not the low-noise amplifier and to direct received signals from the antenna to the low-noise amplifier and not to the power amplifier. Herein it can be problematic to reflect high input power levels from the low noise amplifier back through the circulator and to the power amplifier. This problem can be solved by routing the reflected signal to a high-power load by adding an extra circulator. This effectively creates an absorptive limiter but results in a bulky and costly design.